The invention relates to a method for the continuous control of the delivery rate of reciprocating compressors by occasional holding-open of the suction valves by means of a lifting device, which is actuated by a lifting force supplied by a pressure medium. Furthermore, the invention relates to a device for carrying out the method, with a compressor installation, which comprises at least one cylinder with a lifting device for the suction valve, which is actuated by a lifting force applied by a pressure medium. The magnitude of the lifting force can be determined by a controller.
Control methods of this type and devices for carrying out the methods are already known in several constructions and described, for example, in AT-PS 187 616. In these methods, known as "return flow control," a part of the gas sucked in at the time of the compression stroke of the compressor is pushed back again into the suction line due to the suction valve which is held open positively. The suction valve is closed first as soon as the return flow forces exerted on the closure member of the suction valve by the gas flowing back overcome the holding-open force applied from outside. The compression and consequently the conveyance of the medium begin, upon each compression stroke, only after closing of the suction valve. The holding-open force is applied by a pressure medium in the form of a lifting pressure by way of the lifting device to the closure member of the suction valve. By varying the lifting pressure, the respective delivery rate can be chosen, varied and adapted to the respective requirement.
These controls operate substantially without power losses and allow an infinite variation of the delivery rate. Their control range is however limited at the bottom end due to the operation of the reciprocating compressor. With a certain throttled delivery rate, the return flow forces occurring are no longer sufficient to close the suction valve against the lifting force. The compressor then passes abruptly into the idling state. In practice, the lower limit of the control range of such return flow controls lies at approximately 20 to 40% of the full delivery rate. The sudden shutting-off of the delivery is undesirable, since it may lead to considerable fluctuations of the pressure and of the compressed gas quantity in the pressure system supplied and may cause adverse oscillations, which in turn places a pulsating load on the drive motor and in the case of an electrical drive leads to current surges in the mains.
In order to remedy this, it is known to limit the lifting force to a certain value, which is lower than the lifting force necessary for shutting off the delivery. This limit can be ascertained empirically in a relatively simple manner or even calculated. However, it depends largely on the operating conditions of the compressor and on its design, in addition to the construction, above all on the nature of the medium delivered, in particular on its density. It follows from this that the maximum admissible lifting force can in each case be preset in an optimum manner solely for a certain type of gas and a certain suction pressure.
If the nature of the gas, for example its molecular weight, or the suction pressure vary during the operation, the shut-off barrier no longer functions. Since the lifting force necessary increases with the gas density, the limit value can always be coordinated solely with the resulting gas with the lowest density. However, in the case of delivery of a gas of greater density, the full control range can no longer be utilised, because a lifting force higher than the preset limit value would be necessary for this.
In principle it is possible to measure the gas density in the suction state continuously and to adapt the limit value for the lifting force, during operation, continuously to the measured values. However, determining the gas density is complicated and therefore uneconomical. It depends on the intake pressure, the intake temperature and on the molecular weight of the gas. The intake pressure and temperature are simple to measure, but the measurement of the molecular weight is substantially more complicated. The additionally required linkage of the measured values determining the gas density leads to further complications. In practice, this measure is therefore used only seldom.
It is the object of the invention to provide an improved control method of the aforementioned type, in which the control range can be largely utilised independently of the molecular weight of the respective gas delivered, so that the control method can be used advantageously even when delivering gases having a different or varying molecular weight.